1. Field of the Invention
The present invention relates to a power output apparatus for outputting power to a drive shaft and a vehicle including drive wheels connected to a driveshaft.
2. Description of the Prior Art
Japanese Patent Laid-open No. 2004-175320 discloses a power output apparatus including an infinitely variable transmission (IVT) constituted by a combination of a continuously variable transmission and a planetary gear mechanism, and used as a hybrid vehicle driving apparatus. The power output apparatus includes a motor, a continuously variable transmission, a planetary gear mechanism including a sun gear as a first input element, a carrier as a second input element, and a ring gear as an output element, a high clutch that engages and disengages the sun gear of the planetary gear mechanism with and from an output shaft of the apparatus, and a low clutch that engages and disengages the ring gear of the planetary gear mechanism with and from the output shaft of the apparatus. An input shaft of the continuously variable transmission is connected to an engine and connected to the carrier of the planetary gear mechanism via a parallel gear train. An output shaft of the continuously variable transmission is connected to the sun gear of the planetary gear mechanism and the motor.
In the power output apparatus, a torque circulation mode is set in which the high clutch is disengaged and the low clutch is engaged to cause torque circulation in the continuously variable transmission. In the torque circulation mode, a transmission state of the continuously variable transmission is changed from an acceleration state to a deceleration state to change the sun gear from a high speed (over drive) rotation state at an input speed ratio Ai to a low speed (under drive) rotation speed at an input speed ratio Bi, thereby allowing a speed ratio of the ring gear connected to the output shaft of the apparatus to be changed from a negative speed ratio Ao (reverse state) to a certain speed increasing ratio Bo. In the torque circulation mode, torque from the motor is amplified by the continuously variable transmission, and thus the output shaft can output high torque and a rotation speed of the motor becomes higher than a rotation speed of the output shaft of the apparatus, thereby allowing energy regeneration by the motor in a rotation region with high regenerative efficiency. Further, the power output apparatus disengages the low clutch and engages the high clutch at the time of synchronous rotation between the sun gear and the ring gear to set a direct torque transmission mode. In the direct torque transmission mode, the transmission state of the continuously variable transmission can be changed from a constant speed state to the acceleration state to change a speed ratio of the sun gear as the output element, that is, the output shaft of the apparatus from a constant speed ratio Ci to a high speed ratio Di. In the direct torque transmission mode, torque from the motor can be transmitted to the output shaft without via the continuously variable transmission, thereby allowing transmission efficiency of the motor torque to be increased, and allowing energy regeneration by the motor without a loss in the continuously variable transmission.
In the above described power output apparatus, the torque circulation mode is set to allow high torque to be efficiently outputted to the output shaft in a low speed region. However, in the direct torque transmission mode, only at least one of power from the engine that is changed in speed by the continuously variable transmission and power from the motor can be outputted to the output shaft. Thus, in terms of increasing a range of the transmission gear ratio to increase energy efficiency and torque characteristic in a broad operation region from a low speed region to a high speed region, there is still need for improvement in the conventional power output apparatus.